The present invention relates to polyol/polyisocyanate coating compositions and more particularly to a unique catalyst system effective therefor.
Vapor permeation curable coatings traditionally are a class of coatings formulated from aromatic hydroxyl-functional polymers and multiisocyanate cross-linking agents wherein an applied film thereof is cured by exposure to a vaporous tertiary amine catalyst. In order to contain and handle the vaporous tertiary amine catalyst economically and safely, curing chambers were developed. Curing chambers typically are substantially empty boxes through which a conveyor bearing the coated substrate passes and in which the vaporous tertiary amine, normally borne by an inert gas carrier, contacts such coated substrate. The use of aromatic hydroxy-functional polymers is recommended if an extended pot life system is required. If two-pack formulations are acceptable, then use of aliphatic hydroxyl-functional resins can be made. Multi-isocyanate cross-linking agents in traditional vapor permeation curable coatings contain at least some aromatic isocyanate groups in order for practical cure rates to be achieved.
Such traditional vapor permeation curable coatings requirements have been altered to a degree by the vaporous amine catalyst spray method disclosed by Blegen in U.S. Pat. No. 4,517,222. Such vaporous catalyst spray method relies on the concurrent generation of an atomizate of a coating composition and a carrier gas bearing a catalytic amount of a vaporous tertiary amine catalyst. Such generated atomizate and vaporous catalytic amine-bearing carrier gas flow are admixed and directed onto a substrate to form a film thereover. Curing is rapid and use of a curing chamber is not required. Moreover, all aliphatic isocyanate curing agents can be utilized in such spray process. Aromatic hydroxyl groups on the resin, however, still are required.
One drawback to the requirement of aromatic hydroxyl groups on the resin is the inherent limitation which such aromaticity provides in formulating high solids coatings. The same is true of the requirement of aromaticity in the multi-isocyanate cross-linking agent. Such non-volatile solids content restriction even applies to the vaporous amine catalyst spray method described above.
Despite the improvements in the vapor permeation curable coatings field, all-aliphatic, high-performance urethane top coats still have yet to be developed. Instead, such urethane top coats traditionally are heat-cured in the presence of a tin or like metal catalyst. There is a need in the art for adapting vapor permeation curable coatings technology to such urethane top coats desirably utilizing conventional tin catalyst systems. Such need in the art is addressed by the present invention.